Estimating frequency offset

ABSTRACT

Frequency offset estimation apparatus for estimating the offset from a predetermined centre frequency of an input signal carrying a plurality of frequency shifted symbols, the apparatus comprising: a demodulator for demodulating the input signal to estimate the symbols; a first filter for forming a first estimate of the offset by determining the average of a first predetermined number of the last maxima and minima of the instantaneous frequency difference between the input signal and a signal at the centre frequency; a second filter for forming a second estimate of the offset by determining the average of the values of the instantaneous frequency difference between the input signal at the centre frequency associated with the estimation by the demodulator of those of the symbols having the greatest positive and negative frequency shifts; and selector for selecting the first estimate or the second estimate as an output estimate of the frequency error.

CROSS REFERENCE TO RELATED APPLICATION

This application is a 35 U.S.C. §371 National Phase Entry Applicationfrom PCT/GB02/00683, filed Feb. 15, 2002, and designating the U.S.

This invention relates to estimating frequency offset or error, forexample to permit subsequent correction of the estimated error. Thepresent invention is particularly applicable for correcting error so asto improve the accuracy of demodulation, for example in a radioreceiver.

BACKGROUND OF THE INVENTION

FIG. 1 is a schematic diagram of one form of radio receiver. Thereceiver comprises an antenna 1 connected to an amplifier 2 whichamplifies the radio frequency signal received by the antenna. The outputof the amplifier is filtered by a bandpass filter 3. The filteredsignal, still at radio frequency, is then downconverted by mixing inmixer 4 with a signal generated by oscillator 5. In the receiver of FIG.1 the mixer 4 downconverts the radio frequency signal down to anintermediate frequency (IF). The intermediate frequency signal is thendownconverted to another intermediate frequency or to baseband by mixingin mixer 6 with a signal generated by oscillator 7. Bandpass filters 8and 9 are used between mixer 4 and mixer 6, and after mixer 6. The IF orbaseband signal is passed to demodulator 10 for demodulation todetermine the symbol stream encoded in the signal.

One form of modulation is frequency shift keying (FSK). In FSKmodulation symbols are encoded into the signal as variations (shifts) inthe signal's frequency about a centre frequency. Especially when thesignal is modulated using FSK modulation it is important that thereceived signal as input to the symbol estimator port of the demodulatoris not displaced in its centre frequency with respect to the nominalvalue of the carrier frequency. Otherwise, the possibility of signalsbeing decoded incorrectly is increased. Such a displacement may arise,for example, from errors in the transmission frequency or in the mixingfrequencies used in the receiver (e.g. as generated by oscillators 5 and7).

In order to cope with displacement of the centre frequency of thereceived signal, the demodulator may estimate the displacement and applycompensation to the signal or adjust the, demodulation process with theaim of avoiding demodulation errors due to the displacement.

SUMMARY OF THE INVENTION

When data is sent to the receiver in bursts, for example as packets,rather than continuously, there are potentially conflicting requirementsin the estimation of the displacement of the received signal's centrefrequency. At the start of a received packet it would be preferred toaverage the displacement over a short time period so as to lock quicklyon to an estimated value for the displacement and allow the demodulationprocess to start quickly. This calls for the use of a short timeconstant filter in the estimation process. As more of the packet isreceived it would be preferable to average the displacement over alonger time period, so as to reduce noise in the estimation process andgive a more accurate estimate of the displacement. This calls for theuse of a longer time constant filter.

According to the present invention there is provided frequency offsetestimation apparatus for estimating the offset from a predeterminedcentre frequency of an input signal carrying a plurality of frequencyshifted symbols, the apparatus comprising: a demodulator fordemodulating the input signal to estimate the symbols; a first filterfor forming a first estimate of the offset by determining the average ofa first predetermined number of the last maxima and minima of theinstantaneous frequency difference between the input signal and a signalat the centre frequency; a second filter for forming a second estimateof the offset by determining the average of the values of theinstantaneous frequency difference between the input signal and a signalat the centre frequency associated with the estimation by thedemodulator of those of the symbols having the greatest positive andnegative frequency shifts; and a selector for selecting the firstestimate or the second estimate as an output estimate of the frequencyerror.

Suitably the first predetermined number is greater than 1. Suitably thefirst predetermined number is less than 10, and preferably less than 6.The first predetermined number is most preferably 2, 3 or 4.

The second filter may comprise: a first infinite impulse response filterfor determining a first average of the values of the instantaneousfrequency difference between the input signal and a signal at the centrefrequency associated with the estimation by the demodulator of those ofthe symbols having the greatest positive frequency shifts; a secondinfinite impulse response filter for determining a second average of thevalues of the instantaneous frequency difference between the inputsignal and a signal at the centre frequency associated with theestimation by the demodulator of those of the symbols having thegreatest negative frequency shifts; and an averaging unit fordetermining the average of the first average and the second average.

The filters may be applied to a single instantaneous difference persymbol, corresponding to the sampling time of each symbol, or may beapplied to multiple instantaneous differences during each symbol.

The signal may be encoded using only two symbols.

In the case of a modulation scheme using P different frequencydeviations where P is greater than two, the second filter may comprise Pinfinite impulse response filters with each infinite impulse responsefilter being directed to estimating the instantaneous frequencydifference for one of those P symbols. The outputs of the P filters canthen be combined and/or selected between to give the estimate offrequency error.

The frequency offset estimation apparatus may comprise a third filterfor forming a third estimate of the offset by determining the average ofa second predetermined number of the last maxima and minima of theinstantaneous frequency difference between the input signal and a signalat the centre frequency. The second filter may be arranged to take thethird estimate as an initialisation value for the first and secondinfinite impulse response filters.

The second predetermined number is preferably greater than the firstpredetermined number. The second predetermined number is preferably fouror more.

Suitably, on receipt of a burst of data the demodulator is arranged tosynchronise to the input signal and provide a synchronisation signalindicating whether synchronisation has been achieved.

The second filter may be responsive to the synchronisation signalindicating that synchronisation has been achieved to initialise thefirst and second infinite impulse response filters.

The selector may be responsive to the synchronisation signal to selectthe first estimate if the synchronisation signal indicates thatsynchronisation has not been achieved and to select the second estimateif the synchronisation signal indicates that synchronisation has beenachieved.

The demodulator may comprise: a frequency shifting arrangement forshifting the frequency of the input signal by an amount corresponding tothe output estimate of the frequency error to generate a frequencyshifted signal, and a symbol estimator for estimating the symbols in thefrequency shifted signal. Such a shift would be in a direction such asto negate the estimated frequency error in the input signal.Alternatively, the demodulator may comprise a sensor for sensing theinstantaneous frequency of the input signal and generating a sensedfrequency signal representing the sensed frequency; a frequency shiftingarrangement for receiving the sensed frequency signal and forming afrequency shifted signal representing the sensed frequency shifted by anamount corresponding to the output estimate of the frequency error; anda symbol estimator for estimating the symbols in the frequency shiftedsignal. Alternatively, the demodulator may comprise a symbol estimatorfor estimating the symbols in the input signal by comparing theinstantaneous frequency of the incoming signal with a plurality offrequency thresholds; and a threshold frequency shifting arrangement forshifting the thresholds by an amount corresponding to the outputestimate of the frequency error. Such as shift would be in a directionsuch as to compensate for the estimated frequency error in the inputsignal.

The frequency offset estimation apparatus may be provided in a radiosignal receiver. The radio signal receiver may comprise: frequencyoffset estimation apparatus as claimed in any preceding claim; anantenna for receiving a radio frequency signal; and downconversion meansfor downconverting the radio frequency signal to another intermediatefrequency or to baseband to form the input signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described by way of example withreference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a radio receiver for FSK signals, whichis applicable to the present invention;

FIG. 2 is a schematic diagram of a radio receiver according to oneembodiment of the present invention;

FIG. 3 is a schematic diagram of a radio receiver according to anotherembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the receiver of FIG. 2, the antenna 1, filters 2, 8 and 9, mixers 4and 6 and oscillators 5 and 7 are as described with reference to FIG. 1.Demodulator 10 is of a novel form, as described below.

In the receiver of FIG. 2 a sensor unit 11 is provided for sensing theinstantaneous frequency of the signal as input to the demodulator. Theinput of the sensor unit 11 is connected to the input 13 to thedemodulator. The sensor unit 11 could be a frequency discriminatorcircuit. The sensor unit 11 provides an output at 14 which is indicativeof the instantaneous frequency of the signal input to the demodulator.

The output 14 of the sensor unit 11 is applied to three filters 15, 16and 17. Filter 15 is a short time constant filter. Filter 16 is anintermediate time constant filter. Filter 17 is a longer time constantfilter.

The demodulator includes a frequency compensator 18 which applies afrequency compensation to the output from the sensor unit 11 so as toform a compensated signal at 19. The compensation applied by thecompensator is dependant on the control signal input to it at 20 fromthe filters 15, 16, 17.

The output 19 of the frequency compensator is applied to a symbolestimator 24, symbol timing recovery unit 22 and a synchronisation unit26. The synchronisation unit synchronises the demodulation section 21 tothe received signal. The synchronisation unit 26 provides an output at27 which indicates whether synchronisation has been achieved. The symboltiming recovery unit recovers the timing of symbols in the signal at 19and provides a corresponding output to the symbol estimator 24. Thesymbol estimator 24 recovers symbols from the compensated signal at 19by comparing the frequency represented by the compensated signal withone or more thresholds to establish which symbols are represented by thereceived signal. The output of the symbol estimator 24 is the symbolstream output from the demodulation section at 25. In this example, theresult is provided as one of two symbols represented as a ‘0’ bit or a‘1’ bit in the output at 25, depending on whether the frequency of thereceived signal is higher or lower than a threshold. Other modulationschemes including more than two symbols could be used.

In some implementations the frequency estimator 18 could conveniently beintegrated with the symbol estimator 24. For example, the symbolestimator could simply compare the output of the frequency sensor 11with a threshold that is derived from the frequency error estimate 20 inorder to determine the symbol value.

When a packet or burst of symbols is begun to be received by thereceiver the first symbols of the packet are used by the receiver tosynchronise to the packet. In many systems in which data is transmittedin packets each packet begins with a series of symbols that act as anaccess code to which the receiver can synchronise by means ofsynchronisation unit 26.

Filter 15 is used to estimate the frequency error before the receiverhas synchronised to the access code at the start of each packet. Filter15 includes a difference unit 150, a maxima/minima unit 151 and anaveraging unit 152. Difference unit 150 determines the differencebetween the output from the sensor unit 11, which indicates theinstantaneous frequency of the signal as received, and the nominalcarrier frequency. Note that the nominal carrier frequency maycorrespond to a value of zero, in which case the differencing unit isnot required. The difference signal output from difference unit isapplied to maxima/minima unit 151. The maxima/minima unit 151 determinesthe maxima and minima of the difference signal between each zerocrossing of the difference signal. When the difference signal goesnegative the maxima/minima unit determines its minimum value until itnext goes positive. When the difference signal goes positive themaxima/minima unit determines its minimum value until it next goesnegative. The maxima/minima unit stores the N last maxima and minima instore 153, where N is a predetermined number. N is suitably relativelysmall, for example 2, 3 or 4.

In more detail, maxima minima unit includes a maximum register 154, aminimum register 155 and zero crossing detector 156. Maximum register154 receives the difference signal output from difference unit 150,compares it with a value currently stored in the maximum register andreplaces the value currently stored in the maximum register with thevalue of the difference signal if the difference signal is greater thanthe value currently stored in the maximum register. Minimum register 155receives the difference signal output from difference unit 150, comparesit with a value currently stored in the minimum register and replacesthe value currently stored in the minimum register with the value of thedifference signal if the difference signal is less than the valuecurrently stored in the maximum register. Zero crossing detector 156monitors the difference signal output from difference unit 150 and if itdetects that the signal has crossed zero going from positive to negativeit stores the value in the maximum register in a maximum section 157 ofstore 153 and resets the maximum register to zero. If it detects thatthe signal has crossed zero going from negative to positive it storesthe value in the minimum register in a minimum section 158 of store 153and resets the minimum register to zero. The sections 157 and 158 may beimplemented a FIFO (first in, first out) buffers each of length N sothat between them they store the last N maxima and minima.

Once N maxima and minima have been stored in store 153 averaging unit152 determines the mean of those values. That value is output fromfilter 15 at 159 as an estimate of the frequency error.

The value N is chosen to be fairly small, for example 2, 3 or 4, so thatreceived signal preceding the packet that is being demodulated does notsignificantly influence the estimate of the frequency error generated byfilter 15, and so that the filter 15 can track rapid frequency driftsince frequency drift is typically at its worst at the start of eachpacket. Also, the frequency error estimator must rapidly acquire theerror at the start of each packet since the error can be quite large;subsequently it can track drift in the error at a slower rate.

Filter 16 is similar to filter 15. Components 160 to 169 correspond tocomponents 150 to 159 respectively, with the exception that the maximumand minimum store sections 167 and 168 store a greater number M of lastmaxima and minima, and averaging unit 162 correspondingly averages overthe last M maxima and minima as stored in those store sections. M isgreater than N. Typical values for M are in the range from 4 to 10.Filter 16 provides more averaging of the frequency error—that is over alonger time constant—but is more influenced by received signal precedingthe packet that is being decoded.

Instead of duplicating components between filters 15 and 16, filter 15could be implemented by providing an averaging unit equivalent to unit152 that instead averages the last N values stored in maximum andminimum store sections 167 and 168.

Filter 17 comprises two first order IIR (infinite impulse response)filters 171, 172, a control unit 173, an averaging unit 174 and adifference unit 175. Filter 17 receives the output 169 of filter 16, thesignal at 27 indicating that synchronisation has been achieved. Thecontrol unit activates the filters 171 and 172 when the signal at 27indicates that synchronisation has been achieved. Then the control unitinitialises the filters 171, 172 by setting their initial values to theestimate of the frequency error as output at 169 from filter 16. Thedifference unit 175 determines the difference between theinstantaneously detected received frequency and the nominal frequency.The difference determined at the moment when each ‘0’ bit is detectedaccording to the symbol timing recovery block 22 is applied to IIRfilter 171. The difference determined at the moment when each ‘1’ bit isdetected is applied to IIR filter 172. Thus filter 171 averages thefrequency offsets that correspond to the detection of ‘0’ bits whilstfilter 172 averages the frequency offsets that correspond to thedetection of ‘1’ bits. The outputs of these filters are averaged byaveraging unit 175 to form the output 176 of filter 172 which representsan estimate of the frequency error.

The initialisation of the values for filters 171, 172 can be performedin a number of ways. In one method, the IIR filter estimating thepositive frequency deviation is initiated with the average of the Mmaxima values while the IIR filter estimating the negative frequencydeviation is initiated with the average of the M minima values.Alternatively, the IIR filter estimating the positive frequencydeviation can be initiated with the frequency error plus an assumedfrequency deviation while the IIR filter estimating the negativefrequency deviation can be initiated with the frequency error minus anassumed frequency deviation. The assumed frequency deviation may bedetermined based on the modulation scheme.

The difference units illustrated separately as 150, 160 and 173 could beimplemented as a single unit.

In operation of the receiver the filters 15 and 16 operate continuouslyto provide up-to-date estimates at 159 and 169 of the frequency error.Filter 17 is activated when synchronisation is achieved for the packetcurrently being received, and otherwise filters 171 and 172 provide nooutput.

The estimates generated by filters 15 and 17 at 159 and 176 respectivelyare applied to multiplexer 28. Multiplexer 28 is switched by the signalat 27 indicating that synchronisation has been achieved. The output ofmultiplexer 28 provides the control input at 20 to the frequencycompensator 18. Before synchronisation has been achieved for the presentpacket multiplexer 28 provides the signal from filter 15 at 159 as thecontrol signal to the frequency compensator 18. When synchronisation hasbeen achieved the multiplexer provides the signal from filter 17 at 176as the control signal to the frequency compensator.

The control signal to the frequency compensator 18 indicates anestimated offset of the centre frequency of the received signal from thenominal carrier frequency. The frequency compensator applies acorresponding correction to the signal at 13.

Instead of applying a shift to the detected frequency of the incomingsignal after the signal's frequency has been detected other methodscould be used. For example, the frequency thresholds used by symbolestimator 24 could be shifted in opposite correspondence to the shiftthat would be applied to the incoming signal; in that case the output 20would be applied to symbol estimator 24 and the frequency compensator 18could be omitted, as shown in FIG. 3.

The applicant draws attention to the fact that the present invention mayinclude any feature or combination of features disclosed herein eitherimplicitly or explicitly or any generalisation thereof, withoutlimitation to the scope of any of the present claims. In view of theforegoing description it will be evident to a person skilled in the artthat various modifications may be made within the scope of theinvention.

1. Frequency offset estimation apparatus for estimating the offset froma predetermined nominal carrier frequency of an input signal carrying aplurality of frequency shifted symbols, the apparatus comprising: ademodulator for demodulating the input signal to estimate the symbols; afirst filter for forming a first estimate of the offset by determiningthe average of a first predetermined number of each of the last maximaand last minima of the instantaneous frequency difference between theinput signal and the nominal carrier frequency; a second filter forforming a second estimate of the offset by determining the average ofthe values of the instantaneous frequency difference between the inputsignal and the nominal carrier frequency associated with the estimationby the demodulator of symbols in the input signal; and a selector forselecting the first estimate or the second estimate as an outputestimate of the frequency error.
 2. Frequency offset estimationapparatus as claimed in claim 1, wherein the first predetermined numberis 2, 3, 4 or
 5. 3. Frequency offset estimation apparatus as claimed inclaim 1, wherein the second filter comprises: a first infinite impulseresponse filter for determining a first average of the values of theinstantaneous frequency difference between the input signal and thenominal carrier frequency associated with the estimation by thedemodulator of symbols in the input signal having positive frequencyshifts; a second infinite impulse response filter for determining asecond average of the values of the instantaneous frequency differencebetween the input signal and the nominal carrier frequency associatedwith the estimation by the demodulator of symbols in the input signalhaving negative frequency shifts; and an averaging unit for determiningthe average of the first average and the second average.
 4. Frequencyoffset estimation apparatus as claimed in claim 1, comprising a thirdfilter for forming a third estimate of the offset by determining theaverage of a second predetermined number of each of the last maxima andlast minima of the instantaneous frequency difference between the inputsignal and the nominal carrier frequency; and wherein the second filteris arranged to take the third estimate as an initialisation value forthe first and second infinite impulse response filters.
 5. Frequencyoffset estimation apparatus as claimed in claim 4, wherein the secondpredetermined number is greater than the first predetermined number. 6.Frequency offset estimation apparatus as claimed in claim 1, wherein onreceipt of a burst of data the demodulator is arranged to synchronise tothe input signal and provide a synchronisation signal indicating whethersynchronisation has been achieved.
 7. Frequency offset estimationapparatus as claimed in claim 6, wherein the second filter is responsiveto the synchronisation signal indicating that synchronisation has beenachieved to initialise the first and second infinite impulse responsefilters.
 8. Frequency offset estimation apparatus as claimed in claim 4,wherein the selector is responsive to the synchronisation signal toselect the first estimate if the synchronisation signal indicates thatsynchronisation has not been achieved and to select the second estimateif the synchronisation signal indicates that synchronisation has beenachieved.
 9. Frequency offset estimation apparatus as claimed in claim1, wherein the demodulator comprises: a sensor for sensing theinstantaneous frequency of the input signal and generating a sensedfrequency signal representing the sensed frequency; a frequency shiftingarrangement for receiving the sensed frequency signal and forming afrequency shifted signal representing the sensed frequency shifted by anamount corresponding to the output estimate of the frequency error; anda symbol estimator for estimating the symbols in the frequency shiftedsignal.
 10. Frequency offset estimation apparatus as claimed in claim 1,wherein the demodulator comprises: a symbol estimator for estimating thesymbols in the input signal by comparing the instantaneous frequency ofthe incoming signal with a plurality of frequency thresholds; and athreshold frequency shifting arrangement for shifting the thresholds byan amount corresponding to the output estimate of the frequency error.11. A radio signal receiver comprising: frequency offset estimationapparatus as claimed in claim 1; an antenna for receiving a radiofrequency signal; and downconversion means for downconverting the radiofrequency signal to an intermediate frequency or baseband to form theinput signal.